In a conventional steering system such as for outboard motors used on boats, the propulsion unit or engine typically mounted on the transom of the boat is pivoted about a vertical steering axis upon steering actuation by the operator at the helm. One typical steering system for a boat having a transom mounted engine comprises a steering cable, such as a push-pull cable, extending between the steering helm and the propulsion unit so that steering at the helm actuates the cable for causing steering movement of the propulsion unit about a steering axis. One end of the cable is actuably connected to the steering helm, and the other end is actuably connected to the steering mechanism of the propulsion unit. When the wheel is turned at the helm, the cable is actuated thereby causing a steering movement of the propulsion unit. Hydraulic activated steering means can be used in place of the cable steering, wherein hydraulic fluid, e.g. oil, is pumped from the steering helm through conduits to a cylinder-piston control means in response to rotation of the steering wheel in one direction or the other. Actuation of the control means actuates the steering mechanism of the propulsion unit, thereby turning the propulsion unit in a common direction.
The prior art also teaches power steering systems for an outboard utilizing a hydraulic cylinder-piston assembly and push-pull cables to selectively extend and retract the piston rod and thereby effect steering of the propulsion unit. Typically, such power steering systems are mounted onto and supported by the propulsion unit, which is disadvantageous because special bracketry is required, the supply lines are subject to exposure and abuse, and most of these systems are designed to continuously supply fluid to the system, and not just when steering movement occurs which wastes propulsion engine horsepower.
Power steering systems for marine vehicles mounted remote from the propulsion unit, and overcoming the several disadvantages of the prior art, are disclosed in U.S. Pat. No. 5,228,405, and in co-pending U.S. patent application Ser. No. 08/012,552, both of which are assigned to the same assignee as the subject application, and are incorporated by reference into this specification.
In a hydraulic power steering system, such as of the types described above, torque originating from the propulsion unit is overcome, thereby restraining the steering forces created by this torque. That is, the power steering means reduces the effort at the steering helm or wheel to only the effort required to operate the hydraulic assembly, which is independent of the torque generated by the propulsion unit. There is the possibility, however, that the fluid supply means or source might fail, usually as the result of an electrical failure such as with the battery or some part of the motor. In the event of failure, steering at the helm will quickly deplete or pump out the hydraulic fluid (e.g., pressurized oil) from the cylinder-piston assembly of the system, and because of the failure, the cylinder-piston assembly cannot be maintained or replenished with hydraulic fluid. That is, when the operator turns the wheel, the piston is actuated and hydrualic fluid in the cylinder chamber is forced out or pumped out of the chamber on either the upstroke side of the piston or the downstroke side of the piston, depending on the direction of turn, thereby depleting the hydrualic fluid in the chamber. Because of the failure in the system, there is no means for maintaining hydrualic fluid in the cylinder, or to replenish the cylinder with fluid, so as to maintian a hydraulically locked system. As a consequence, if the fluid is substantially depleted from the cylinder-piston assembly, or essentially no fluid remains, the piston is not actuated or reciprocated by hydraulic pressure, and there is no means for restraining the steering forces or torque created by the propulsion unit except for manual steering. The piston is free to move but has no effect on the steering actuation (the cylinder-piston assembly is not hydraulically locked), and torque originating from the propulsion unit will feed back to the steering helm. The boat can be steered manually, but the torque feed-back requires a strong steering wheel effort to maintain the boat in a straight line or to turn the boat. If a firm grip is not maintained, the boat can suddenly make an abrupt turn in one direction or the other, thereby creating a dangerous and hazardous condition.
This invention has as its purpose to provide in such a power steering system utilizing a hydraulic cylinder, means for maintaining anti-feed back in the event of a failure by preventing loss of hydraulic fluid in the cylinder.